1. Field of the Invention
This invention relates generally to the production of purified aqueous solutions. More particularly, the present invention relates to the electrochemical reduction of oxynitrogen species in aqueous solutions to remove such species by reducing them to environmentally safe forms, such as nitrogen gas or ammonia ions. The process and the apparatus employing the process are suitable for both waste water and potable water treatment. The electrochemical reduction occurs on a high surface area cathode structure. Both dilute and concentrated aqueous solutions of oxynitrogen species can be reduced to environmentally safe nitrogen by the process and apparatus of this invention.
2. Description of Related Art
Nitrogen-based fertilizers that are widely used in agriculture cause nitrite and nitrate pollution of ground water by rain water run off from farm fields. Manufacturing operations which utilize nitric acid can also produce waste streams containing nitrites and nitrates, as well as gaseous emissions containing oxygenated nitrogen gas species. These gaseous emissions can result in acid rainfall in the form of nitric acid. The presence of nitrites and nitrates in drinking water is undesirable because of their hazards to human health. As such, the maximum allowable concentrations of nitrites and nitrates in drinking water is limited by current governmental regulations to 10 parts per million. Much attention has recently been focused on drinking water treatment to remove nitrates and nitrites in both the United States and in Europe.
Prior approaches to remove or reduce nitrates and nitrites from aqueous streams require the use of either a chemical agent or an expensive process typically utilizing ion exchange, reverse osmosis, electrodialysis or distillation. Ion exchange methods require regeneration of the ion exchange resin beds or zeolites with salts or with acids or bases. The spent regenerants must be disposed of and cause further pollution and waste disposal problems. Such prior chemical methods are expensive, can require costly capital expenditures for processing equipment and can still create waste disposal problems. U.S. Pat. Nos. 4,956,057 and 4,990,266 describe recent approaches utilized to remove the nitrate/nitrite contaminants from aqueous streams. The former uses electrolytic reduction at the cathode to produce a gas containing H.sub.2, NH.sub.3, NO and N.sub.2 O, which is then passed through a catalyst bed for reaction to produce N.sub.2 and H.sub.2 O. The latter chemical method utilizes catalytic hydrogenation with a palladium and/or rhodium or palladium and copper group metal catalyst impregnated into a porous metal carrier to produce nitrogen gas in a continuous catalytic reduction process in which the pH of the water is maintained at a level not greater than 8.
Biological approaches to destroy or reduce nitrates are also widely employed, but such approaches are generally slow, have variable results, and require large amounts of capital investment and large tracts of land.
These and other problems are solved by the process of the present invention which removes both nitrites and nitrates, as well as other oxynitrogen species, from aqueous solutions by the use of a specific electrochemical reduction process using a particular high surface area cathode.